Unless the computer is going to be a Database server or online Web Server anything over 4 core is a waste. If he is doing design work (I assume photoshop) even 4 cores is overkill. My i7 Mac Book Pro with 2 cores and hyper threading is just as fast or Faster than a friends quad Mac Pro faster actually. Writing apps to effectively use more than a couple of Cores is not a trivial task. A Quad core with lots of Ram is much better from a price performance perspective, now if it's a database server or web server where you are spawning lots of threads then the 8 core boxes are worth the premium, or if you are running multiple virtual machines. Having more than 4 cores on a desktop workstation is great for bragging rites but not much more!
BT
Just my two cents, often only worth a Penny!
1) If your using Final Cut Studio, anything below a Quad Core is ridiculously slow, and you need 8+ to make it run.
2) No, but I cant multitask as much on my Core 2 Duo MacBook as I can on my lovely Mac Pro Quad (roughly equivalent to the 8-Core mentioned here in terms of performance due to mine not having a FSB and having the new architecture)
3) 16 year olds do a lot at once, Im sure he could push a Quad to its knees if he really really tried (of that era).
4) You are fundamentally wrong when you say you only need 4 cores on the desktop. If you try to render videos, having 4 threading to 8 is enough, until you go the HD, then even my Mac Pro 5,1 Quad seems slow and I wish I could have 8 - 12 cores to deal with rendering 1.5hrs of HD footage. There are plenty of applications, both within the creative industry (Video Editors, Video Producers, People who do a lot at once - such as myself (Average App Load: Mail, Safari, iTunes, XCode, Final Cut Pro, Logic Pro, Motion 4, Soundtrack Pro, Dreamweaver, Preview, iChat, Skype, Terminal, Console) - heck it slowed my Mac Pro down enough I bought another Mac just to run iTunes.) and within the Sciences (People who do research for instance) - all of whom cant manage on 2,4 or even 8 cores, however they thread.